Carbon nanotubes (CNTs), with their unique shapes and characteristics, may find various applications. A carbon nanotube has a tubular shape of one-dimensional nature which is obtained by rolling one or more graphene sheets composed of six-membered rings of carbon atoms into a tube. A carbon nanotube formed from one graphene sheet is called a single-wall carbon nanotube (SWNT) while a carbon nanotube formed from multiple graphene sheets is called a multi-wall carbon nanotube (MWNT). SWNTs are about 1 nm in diameter whereas multi-wall carbon nanotubes are several tens nm in diameter, and both are far thinner than their predecessors, which are called carbon fibers.
One of the characteristics of carbon nanotubes resides in that the aspect ratio of length to diameter is very large since the length of carbon nanotubes is on the order of micrometers. Carbon nanotubes are unique in their extremely rare nature of being both metallic and semiconductive depending on the spiral structures because six-membered rings of carbon atoms in carbon nanotubes are arranged into a spiral. Normally, they are obtained as a mixture of metallic and semiconductive carbon nanotubes. In addition, the electrical conductivity of carbon nanotubes is very high and allows a current flow at a current density of 100 MA/cm2 or more.
Carbon nanotubes excel not only in electrical characteristics but also in mechanical characteristics. That is, the carbon nanotubes are distinctively tough, as attested by their Young's moduli exceeding 1 TPa, which belies their extreme lightness resulting from being formed solely of carbon atoms. In addition, the carbon nanotubes have high elasticity and resiliency resulting from their cage structure. Having such various and excellent characteristics, carbon nanotubes are very appealing as industrial materials.
Applied researches that exploit the excellent characteristics of carbon nanotubes have been heretofore made extensively. To give a few examples, a carbon nanotube is added as a resin reinforcer or as a conductive composite material while another research uses a carbon nanotube as a probe of a scanning probe microscope. Carbon nanotubes have also been used as minute electron sources, electric field emission electron devices, and flat displays. An application that is being developed is to use a carbon nanotube as hydrogen storage.
As described above, carbon nanotubes are expected to find use in various applications, and their application as electronic materials and electronic devices has been attracting attention. Electronic devices such as a diode and a transistor have already been prototyped by using carbon nanotubes, and are expected to replace the existing silicon semiconductors.
However, it is extremely difficult to actually wire carbon nanotubes. At present, several techniques of wiring carbon nanotubes have been attempted.
A first technique includes: picking up one or several carbon nanotubes by using a manipulator in a scanning electron microscope; and arranging the one or several carbon nanotubes at a desired position. A technique for arranging carbon nanotubes by using a probe microscope may be given as an example of a modification of the first technique. However, the technique requires much time and labor. Therefore, the technique is suitable for fundamental studies but is not practical.
A second technique is a technique for orienting a carbon nanotube in a certain direction by using electrophoresis. With this technique, carbon nanotubes may be wired in one direction, but it is difficult to wire carbon nanotubes in plural directions. Thus, this technique is not realistic.
A third technique is a technique employing a chemical vapor deposition (CVD) method. The CVD method includes: using an acetylene gas or methane gas containing carbon as a raw material; and producing a carbon nanotube by a chemical decomposition reaction of the raw material gas.
A. Cassell, N. Franklin, T. Tombler, E. Chan, J. Han, and H. Dai, J. Am. Chem. Soc. 121, 7975-7976 (1999) discloses a method of wiring a carbon nanotube horizontally to a substrate. That is, disclosed is a technique including: fabricating a Si pillar on a substrate; mounting an additive on the top part of the pillar; and allowing a methane gas to flow to bridge a carbon nanotube between the pillars. The method by this technique has certainly enabled horizontal wiring. However, the probability of cross-linking is extremely low, and wiring at an arbitrary position is still difficult.
As described above, a technique for wiring one or several carbon nanotubes is still at a developmental stage.
In the meantime, a method for wiring or patterning using a carbon nanotube as a film has been developed. For example, pattern formation of a carbon nanotube has been heretofore performed by using a screen printing method or a photolithography technique. Each of those techniques is excellent in forming a pattern in a wide area at once, and is used for patterning of an electron source in a field emission type display (FED). However, in each of those methods, a carbon nanotube is merely dispersed in a solvent before application, or is mixed with a binder before application. Therefore, the carbon nanotube is insufficient in terms of performance such as a mechanical strength or electrical conductivity, and is hardly used directly as an electrode or an electric circuit.
JP-T-2002-503204 (“JP-T” means searched and published International patent application) discloses that a carbon nanotube with a three-dimensional structure can be formed by using a functionalized carbon nanotube. However, this publication merely discloses, a use of carbon nanotubes which are deposited onto a metal mesh followed by being made porous, for simple use as a chromatography-flow cell electrode. In this case, the carbon nanotube is porous and a functional group is bonded thereto in order to separate and absorb a passing substance. The publication also discloses carbon nanotubes bonded to each other by using an alkoxide of aluminum or silica (the alkoxide itself serves as an insulator) as a cross-linking agent.
However, because the alkoxide crosslinks with itself, the carbon nanotube structure obtained has polymeric alkoxide residues having a cross-linking degree of several dozens randomly forming chains, causing fluctuation of the distance between carbon nanotubes and the chemical structure at the cross-linking sites and consequently prohibiting production of carbon nanotube with intended properties, which in turn imposes various restrictions in use. In addition, the network structure of carbon nanotube formed is not sufficiently dense, and thus, such a carbon nanotube had a problem that it was difficult to use the favorable properties inherent to carbon nanotube such as favorable electrical and thermal conductivity and mechanical strength efficiently.